These STO oscillators are intended to operate in frequency ranges of between a few hundred MHz and a few GHz.
The present invention belongs to the field of spin transfer radiofrequency oscillators, known as STO (“Spin Torque Oscillator”) oscillators.
These STO oscillators are intended to operate in frequency ranges of between a few hundred MHz and a few GHz.
The term oscillator designates a physical system in which at least one of the properties varies in a periodic or quasi-periodic manner over time. The role of an oscillator is to be used as a time and frequency reference.
In particular, oscillators are widely used in the telecommunications field. For example, oscillators of the VCO “Voltage Controlled Oscillator” type are known: These devices are based on a resonant electronic circuit comprising a resistance R, an inductance L and capacitance C, the whole forming an RLC circuit. VCO oscillators not only use this RLC architecture but also have an electrical bias voltage enabling the L and C values to be varied and thereby to modify the frequency.
The electronics of new telecommunications products (mobile telephones for example) must be capable of working over very large frequency ranges. Therefore, VCOs must be capable of transmitting over several frequency ranges. For example, current portable telephones have three or four frequency bands. In addition, the development of mobile technologies imposes additional constraints in terms of the compactness of the products.
Consequently, to meet these demands, either an oscillator with high frequency tunability or agility to cover all frequency ranges must be used or several oscillators must be used. The first solution is the most suitable but is not possible with a single VCO that has too low agility. Consequently, the current solution involves the use of several VCOs, which poses a problem of bulkiness and adds interference phenomena between the various VCOs.
A known solution that is likely to meet the problems mentioned above consists of turning to spintronic device-based radiofrequency oscillators known as STO oscillators.
The operation of these STOs is based on the effects of GMR (“Giant MagnetoResistance”), such as with spin valves, and on the effects of TMR (“Tunnel MagnetoResistance”), such as with MTJ magnetic tunnel junctions.
These structures consist of a stack of magnetic layers, the nature and arrangement of which are made so that when an electrical current traverses them, it is possible to obtain a variable resistance according to the magnetic field applied and/or the spin polarized current that traverses them. Such a device is constituted of a stack of two ferromagnetic layers (one magnetic layer called “pinned,” the magnetization of which is at a fixed direction and one layer called “free,” the magnetization of which is variable) separated by an amagnetic (not magnetic) layer conventionally called a spacer, made in metal for spin valves or in oxide for magnetic tunnel junctions.
In a known manner, when the magnetization orientation of the two ferromagnetic layers is identical, this is then called parallel orientation; the device is in the low resistance state. Consequently, when the orientation of the two ferromagnetic layers is antiparallel, the device is in the high resistance state.
Spin electronics uses the spin of electrons as an additional degree of freedom, in order to generate new effects. The spin of electrons is at the origin of magnetoresistance phenomena in magnetic multilayers, such as, in particular, giant magnetoresistance or tunnel magnetoresistance.
In fact it has been shown that by passing a spin polarized current through a thin magnetic layer and depending on the amplitude of said current, two distinct effects may be induced:                First of all, a reversal of its magnetization in the absence of any external magnetic field; this phenomenon may also be implemented as a means of writing information in the context of producing magnetic random access memories, also called MRAM;        but also, the excitation of sustained precession states of the magnetic moment of the layer: This phenomenon is used in STO.        
Therefore, the operating principle of STO (Spin Torque Oscillator) consists of using the spin transfer torque to trigger sustained magnetization precession. In a magnetoresistive device, this precession causes oscillation of the resistance and therefore generation of alternating voltage in the GHz range. The major advantage of STO oscillators is their high frequency agility since resonance frequency operates over a very wide band depending on the polarization applied to the spintronic device.
The main technical problem posed by STO remains the spectral purity of these oscillators, the bandwidth typically being a few dozen MHz in the best cases (see for example, Mizushima et al, Appl. Phys. Lett. 94, 152501, 2009; Georges et al., Phys. Rev. B 80, 060404(R), 2009). This high bandwidth is due to magnetization trajectory instabilities. This problem is not only connected to thermal fluctuations but is also present at low temperatures (cf. Georges al., Phys. Rev. B 80, 060404(R), 2009).
One of the solutions considered to increase the power and possibly reduce the bandwidth of these STO oscillators is to couple them, either physically on the same device (cf. Kaka et al., Nature 437, 389, 2005), or electrically by connecting them to each other (cf. Georges et al., Appl. Phys. Lett. 92, 232504, 2008). These methods effectively show a coupling of oscillators over a certain frequency range.
However, such a solution poses certain difficulties.
In fact, so that this coupling leads to a synchronization of oscillators, the latter must have very similar properties from the start, which is not easy to achieve from a technological point of view.
In addition, synchronization reduces the frequency tunability of the assembly of coupled oscillators.
In addition, it will be noted that currently, the power emitted by these STOs is relatively weak (at most a few thousand nV2/Hz—cf. for example, Houssameddine et al., Appl. Phys. Lett. 93, 022505, 2008).
A known solution for increasing the radiofrequency signal power produced by spin transfer oscillators is described in U.S. Pat. No. 7,504,898. This document proposes using a separating layer of the CCP-GMR (“Current-Confined-Paths-GMR”) type. CCP-GMR structures are characterized by an insulating layer (sometimes called “current screen layer”) pierced with small holes or conductive bridges (also designated by the term “pinholes”), situated at the center of the magnetic stack, generally between the soft free layer of the GMR structure and the pinned reference layer. The principle described in this document is to take advantage of the high current densities near the conductive bridges to generate magnetization oscillations locally without risking reversal of the magnetization of the soft layer. In fact, in the areas separating the conductive bridges, as the current densities are much lower, the magnetization of these areas is not subject to spin transfer torque and therefore remains almost at rest. Therefore, the excitations generated at the level of each conductive bridge are joined to one another through spin waves. But, between each conductive bridge, an intermediate area remains in which the magnetization is not globally excited. These areas may be traversed by spin waves issued from nearby conductive bridges but do not undergo high amplitude magnetic excitation.
Such a solution also presents certain disadvantages.
Thus, despite an increase in power, the power emitted by STO devices such as described in document U.S. Pat. No. 7,504,898 will remain relatively weak.
In addition, the operating regime of such a device will provide very little harmonic frequency (which may be used in addition to the fundamental frequency).
BRIEF SUMMARY OF THE INVENTION
In this context, the object of the present invention is to provide a spin transfer oscillator presenting, compared to existing STOs, increased emitted power, a good quality factor, very high spectral purity (i.e., narrow fundamental frequency bandwidth), reduced phase noise and the possibility to exploit a plurality of harmonics in order to be able to exploit higher frequencies.
For this purpose, the invention proposes a spin transfer oscillator comprising:                a magnetic stack including at least two magnetic layers in which at least one of said two magnetic layers, called an oscillating layer, has variable direction magnetization;        current supply means capable of causing the flow of a current of electrons perpendicularly to the plane of said magnetic stack;        
Said magnetic stack comprises means capable of generating inhomogeneities of current at the level of the surface of said oscillating layer, said oscillator being characterized in that the intensity of the current provided by said supply means is chosen such that the magnetization of said oscillating layer presents a consistent magnetic configuration, said magnetic configuration oscillating in its entirety at the same fundamental frequency.
Consistent magnetic configuration is understood to refer to a configuration that is close to the magnetic monodomain or that presents magnetization rotations on a length scale comparable to the lateral dimension of the surface of the oscillating layer: It may be a configuration of the magnetic vortex, magnetic antivortex, multivortex or “C-state” type or a combination of these configurations (for example several vortices or one or more vortex-antivortex pairs). By way of examples, such consistent micromagnetic configurations are described in the article “Stability of magnetic vortices in flat submicron permalloy cylinders” (M. Schneider et al, Journ. Appl. Phys. 92 (2002)1466). In the rest of the description, we will talk about a vortex configuration for this consistent magnetic configuration but the person skilled in the art will be able to implement the invention for other types of consistent magnetic configurations.
It will be noted that the oscillating layer may be a simple ferromagnetic layer but also a synthetic antiferromagnetic layer comprising two soft ferromagnetic layers, possibly with different thicknesses, separated by an antiferromagnetic coupling layer.
The oscillator according to the invention is based on collective and consistent excitation dynamics of the magnetization of the entire device (for example the dynamics of a magnetic vortex) induced by the spin transfer torque in a magnetic structure presenting strong lateral inhomogeneities in the distribution of current traversing the structure via means able to generate current inhomogeneities at the level of the surface of the oscillating layer: These inhomogeneities may for example be created in a non-limited manner by a set of conductive nanobridges through an insulating barrier that allows the current lines to be locally concentrated. Typically, these inhomogeneities lead to fluctuations of more than a factor of 10 in conductivity between the most conductive areas and the least conductive areas.
The present invention is set apart from the prior art (and particularly from document U.S. Pat. No. 7,504,898) in that the structure with inhomogeneous current is not used to generate magnetization precessions on the localized areas interacting with each other by spin waves; On the contrary, in a surprising manner, the applicant realized that a current intensity greater than a threshold determined according to the magnetic structure would enable collective excitation dynamics of all of the magnetization of the oscillating layer of the oscillator to be triggered, such as for example vortex type dynamics, the fundamental frequency of which is generally situated at a lower frequency (on the order of a few hundred MHz) than the fundamental frequency of known STOs. Therefore in the present invention, magnetization operates over the entire surface of the excited layer of the oscillator (oscillating layer) leading to a complete mixing of the magnetization of this layer. In other words, the present invention does not seek at all to prevent the magnetization of the excited layer from being reversed, contrary to U.S. Pat. No. 7,504,898 but, on the contrary, seeks to induce a collective excitation movement of the consistent magnetic configuration of magnetization of the excited layer of the oscillator, including the entire surface of the layer. Therefore, the invention is based on the use of spin transfer torque generated by the distribution of inhomogeneous current that, by the averaging effect due to exchange interactions inside the oscillating layer, collectively excites all of the magnetization of this oscillating layer. This state, that simulations demonstrate, is physically confirmed by its consequences such as the high number of harmonics. This is an essential difference from document U.S. Pat. No. 7,504,898, in which the magnetization excitation of the soft layer is produced near pinholes, while the magnetization between pinholes is not or is only weakly excited. In the embodiment of the invention, a large number of harmonics is observed, which is not the case with the operating regime of U.S. Pat. No. 7,504,898. Therefore, according to the invention, at least four exploitable amplitude harmonics are observed, contrary to the operating regime of U.S. Pat. No. 7,504,898.
As will be seen in further detail subsequently, the principle of the invention therefore consists of using a structure with inhomogeneous current to generate simultaneously:                a consistent micromagnetic configuration of the magnetization resulting from the equilibrium between the amperian field produced by the current traversing the structure, the field effects demagnetizing the structure and the exchange interactions within the oscillating layer;        an intense spin transfer effect essentially located at the level of the points of high current density but producing collective excitation of the magnetization over the entire surface of the oscillating layer due to the exchange interactions existing inside the oscillating layer.        
It will be noted that the first effect (appearance of a consistent magnetic configuration) may only intervene for a lower current intensity; one then has a consistent static magnetic configuration; By increasing the current intensity beyond a threshold intensity, the two effects are observed simultaneously (consistent dynamic magnetic configuration).
The use of systems with strongly inhomogeneous current is in an intermediate situation between that of all-metal stacks (spin valve with introduction of current perpendicularly to the plane of the layers) and those with magnetic tunnel junctions. The advantage of the present invention compared to all-metal stacks is to offer a higher resistance times surface area and, correlatively, a higher absolute magnetoresistance and therefore stronger RF signals generated by the excitation caused by the spin transfer. In addition, the advantage of the present invention compared to magnetic tunnel junctions is to pass more current through the structure, of equal cross section, leading to a high amperian field and therefore the possibility to generate a vortex type configuration having consistent dynamics, which could not be obtained with a tunnel junction. In fact in a tunnel junction, the current density is limited by the electric breakdown of the junction. Consequently, the only way to increase the total current through a tunnel junction is to increase its cross section but this leads to a loss of consistency of the magnetization dynamics.
The oscillating movement of the consistent magnetic configuration (typically the cyclical movement of the vortex) thus obtained leads to a cyclic temporal change of the mean magnetization, and consequently to resistance oscillation because of the magnetoresistive effect. Thus alternating voltage may be generated at the terminals of the device, presenting an intense fundamental mode (typically between 300 and 500 MHz) and many harmonics (typically from 12 to 15).
It will be noted that the oscillator according to the invention may operate either with a fixed magnetization layer (pinned layer) and a variable magnetization layer (oscillating layer) or with two oscillating layers (with or without a reference layer), the only condition being that the magnetic stack comprises at least two magnetic layers, one of which is an oscillating layer with variable direction magnetization.
The invention enables an oscillator that is frequency tunable, but with high power (several hundred thousand nV2/Hz or one to two orders of magnitude higher than known STOs) and with very high spectral purity (Q>300, Δf<2 MHz) to be produced. The fundamental frequency is typically between a few hundred MHz and a few GHz. The device according to the invention also generates a large series of harmonics (typically at least four harmonics, the amplitude of which remains less than that of the fundamental frequency but is sufficient to be widely exploitable): Therefore, according to the considered application, the fundamental frequency emitted or some of the harmonics generated by the oscillator may be used for upconverting.
The oscillator according to the invention may also present one or more of the characteristics below, considered individually or according to all technically possible combinations:                said means able to generate current inhomogeneities at the level of the surface of said oscillating layer are formed by a layer presenting strong conductivity inhomogeneities able to generate in said oscillating layer strong lateral current inhomogeneities;        said layer presenting strong conductivity inhomogeneities able to generate in said oscillating layer strong lateral current inhomogeneities is a layer characterized by conductivity fluctuations of more than a factor of 10 between its most conductive areas and its least conductive areas;        said layer presenting strong conductivity inhomogeneities is an insulating layer integrating metal paths;        said layer presenting strong conductivity inhomogeneities is a tunnel junction integrating conductive paths obtained by exposure of said junction to a suitable voltage (i.e., the voltage must be sufficiently high to locally generate electric breakdown phenomena);        said layer presenting strong conductivity inhomogeneities is made from a base of a mixture of two materials with different conductivities able to demix under the effect of annealing to form paths locally that are more conductive than on the rest of the layer; in other words, said layer presenting strong conductivity inhomogeneities is made from a base of a mixture of two immiscible materials having different electrical conductivities so as to form strong electrical conductivity inhomogeneities under the effect of annealing causing the two constituent materials to demix;        said layer presenting strong conductivity inhomogeneities presents a ratio of electric conductivity equal to or greater than 10 between its most conductive areas and its least conductive areas;        said layer presenting strong conductivity inhomogeneities comprises a non-magnetic metallic layer in contact with at least one of its faces;        said means able to generate current inhomogeneities at the level of the surface of said oscillating layer are formed by a plurality of nanocontacts;        said consistent magnetic configuration is a configuration of the magnetic vortex, magnetic antivortex, multivortex or “C-state” type or a combination of these configurations;        said magnetic stack has the form of a pillar, the cross section of which has one of the following forms:                    circular;            elliptical;            annular;            form with axial symmetry comprising at least three lobes;                        the diameter in the case of a circular, annular or flower-form cross section is the larger of two diameters; in the case of a cell with an elliptical form, it is between 10 nm and 500 nm and preferentially between 100 and 500 nm;        said oscillating layer presents a substantially circular cross section with a radius R and thickness L, the L/R ratio being chosen according to the operating frequency of the oscillator, if necessary repeatedly by using micromagnetic simulation software to determine the corresponding frequency at a given L/R value;        said magnetic stack successively comprises:        a magnetic reference layer with a fixed magnetization direction;        a layer with strong lateral electrical conductivity inhomogeneities;        said oscillating magnetic layer with a variable magnetization direction;        said magnetic stack successively comprises two magnetic sub-stacks separated by a magnetic decoupling layer, each of the two sub-stacks integrating:        a magnetic reference layer with a fixed magnetization direction;        a layer with strong lateral electrical conductivity inhomogeneities;        an oscillating magnetic layer with a variable magnetization direction;        said magnetic stack comprises a magnetic sub-stack successively including:        a magnetic reference layer with a fixed magnetization direction;        an amagnetic decoupling layer;        said oscillating magnetic layer with a variable magnetization direction;said means able to generate current inhomogeneities at the level of the surface of said oscillating layer are located outside of said sub-stack;        said magnetic stack comprises two magnetic sub-stacks, each sub-stack successively including:        a magnetic reference layer with a fixed magnetization direction;        an amagnetic decoupling layer;        an oscillating magnetic layer with a variable magnetization direction;        said two sub-stacks being separated by a layer presenting strong conductivity inhomogeneities;        said magnetic stack comprises two oscillating ferromagnetic layers of variable magnetization direction separated by said means able to generate current inhomogeneities at the level of the surface of said oscillating layers;        said magnetic stack comprises:        a magnetic sub-stack successively including:        a first oscillating magnetic layer with a variable magnetization direction;        an amagnetic decoupling layer;        a second oscillating magnetic layer with a variable magnetization direction;        first means able to generate current inhomogeneities at the level of the surface of said first oscillating layer;        second means able to generate current inhomogeneities at the level of the surface of said second oscillating layer;Said first and second means able to generate current inhomogeneities being located outside said sub-stack.        